The long term goal of this project is to identify the cellular and molecular mechanisms that give rise to the synaptic defects in Parkinson?s disease (PD), dementia with Lewy bodies (DLB), and variants of Alzheimer?s disease (AD) and to develop strategies for reversing them. A pathological hallmark of these diseases is aggregation of ?-synuclein throughout the neuron, including synapses. The synaptic aggregation of ?-synuclein is thought to be the cause of the cognitive deficits and dementia. While it is generally agreed that ?-synuclein accumulation at synapses impairs vesicle endocytosis, the underlying mechanisms remain unclear. Thus, at present, there are no known strategies for improving synaptic function in PD, DLB or AD because we don?t know the cellular or molecular targets. The proposed experiments take significant steps toward these goals by taking advantage of two classical vertebrate synapses that are ideally suited for studies on synaptic vesicle trafficking and by using both acute and genetic perturbations of ?-synuclein. The approach includes a combination of quantitative biochemical, electrophysiological, and imaging assays. One model for ?-synuclein toxicity suggests that it is initiated by formation of abnormal oligomers, while another proposes that build up of monomers is the trigger. Aim 1 will test predictions of both models at synapses by identifying how defined molecular species of ?-synuclein (monomers, dimers, higher molecular weight oligomers) affect vesicle trafficking and neurotransmission and the underlying mechanisms. Preliminary studies indicate that monomers and dimers produce distinct effects. Experiments in Aim 2 will determine the role for ?- synuclein self-association in producing synaptic defects by testing reagents that interfere with this process, including a drug with potential therapeutic value. Aim 3 is focused on reversing the synaptic defects caused by excess ?-synuclein by perturbing its association with Hsc70 chaperone protein, an idea that is supported by preliminary data. The experiments are innovative because they are the first to test the effects of defined molecular species of ?-synuclein at synapses, they continue the development of a new model synapse for these studies, and they will test several new reagents with potential for ameliorating the synaptic defects. The experiments are significant because they will elucidate the mechanisms by which excess ?-synuclein causes synaptic deficits, and they will provide possible targeted, molecular strategies for improving synaptic function. Thus, these studies have direct implications for slowing or halting the neurodegeneration, cognitive deficits, and dementia in PD, DLB and other related diseases.